If you mean can CS4 open a CS5 file, no. If you mean can CS5 open a CS4 file (or save as a CS4 file), yes (at least in theory).
The correct word from Ned Murphy answer is 'in theory'. As actually there are lots of bugs when working with cs4 files in cs5:
a. compiler works incorrectly and sometimes throws compile errors where cs4 compiles the same file correctly with no errors; and these issues concern not some 'reserved words' or classes that are omited in cs4, but rather as simple and unexpected elements as movie clips names, etc.
b. when saving FLA file with embedded fonts in cs4 format fonts may include all outlines even though you may select just a few sets; so it may result in compiled SWF with quite huge size (imagine embedding all unicode Arial and Times fonts - that would take about 500 kb on fonts only).
c. one time I had a bug with one graphic asset being replaced with another after compilation; that was I spent about two crazy hours finding out what causes this strange bug and couldn't - after restarting Flash the bug was eliminated in the same strange way.
And that may be not the full list as Flash CS5 has some serious problems with Library - it causes lots of crashes.